Problem: $ D = \left[\begin{array}{rr}1 & 2 \\ 2 & 3\end{array}\right]$ $ A = \left[\begin{array}{rrr}3 & 0 & 3 \\ 3 & 0 & 3\end{array}\right]$ What is $ D A$ ?
Solution: Because $ D$ has dimensions $(2\times2)$ and $ A$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ D A = \left[\begin{array}{rr}{1} & {2} \\ {2} & {3}\end{array}\right] \left[\begin{array}{rrr}{3} & \color{#DF0030}{0} & \color{#9D38BD}{3} \\ {3} & \color{#DF0030}{0} & \color{#9D38BD}{3}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ A$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ A$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ A$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{1}\cdot{3}+{2}\cdot{3} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ A$ and add the products together. $ \left[\begin{array}{rrr}{1}\cdot{3}+{2}\cdot{3} & ? & ? \\ {2}\cdot{3}+{3}\cdot{3} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ D$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ A$ and add the products together. $ \left[\begin{array}{rrr}{1}\cdot{3}+{2}\cdot{3} & {1}\cdot\color{#DF0030}{0}+{2}\cdot\color{#DF0030}{0} & ? \\ {2}\cdot{3}+{3}\cdot{3} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{1}\cdot{3}+{2}\cdot{3} & {1}\cdot\color{#DF0030}{0}+{2}\cdot\color{#DF0030}{0} & {1}\cdot\color{#9D38BD}{3}+{2}\cdot\color{#9D38BD}{3} \\ {2}\cdot{3}+{3}\cdot{3} & {2}\cdot\color{#DF0030}{0}+{3}\cdot\color{#DF0030}{0} & {2}\cdot\color{#9D38BD}{3}+{3}\cdot\color{#9D38BD}{3}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}9 & 0 & 9 \\ 15 & 0 & 15\end{array}\right] $